Surface plasmon resonance (SPR) of metal nanoparticles has opened up emerging opportunities in a broad range of applications, such as chemical or biomolecular sensing, imaging, photothermal therapy of tumors, and sensitization in solar energy harvesting. These applications can rely on large extinction cross-sections of the nanoparticles for improved performance and high chemical stability for extended lifetime, which are highly desired and have been the goal of a long-term exploration.
It has been recognized that the plasmonic property of metal nanoparticles can be largely dependent on their intrinsic dielectric constants apart from their size, anisotropy and property of the media, so that a silver (Ag)-rich composition is essential among various metals for supporting strong surface plasmon polarization modes across the visible range of the electromagnetic spectrum, due to its highest plasmonic activity in terms of quality factor arising from its appropriate electronic structure and thus dielectric function.
Unfortunately, the excellent plasmonic property of Ag nanostructures has not been fully utilized in practical applications due to their poor chemical and structural stability against non-ideal chemical environments. To this end, methods have been developed in earlier studies such that a Ag nanostructure is stabilized for potential plasmonic applications, typically through coverage of an inorganic layer or a self-assembled organic monolayer (SAM) over a Ag nanoparticle, giving rise to a core/shell nanostructure. However, the overlayer is often vulnerable to external etchants, and prolonged exposure to them can still easily cause a loss of the stability of the Ag nanoparticles. In addition, the layer covering Ag nanostructures may often diminish the plasmonic activity of the original Ag nanostructures. Therefore, it becomes crucial to devise alternative strategies for achieving a Ag-rich nanostructure with high stability and excellent plasmonic property, for example, by alloying Ag with a chemically inert plasmonic metal such as gold (Au).
Conventionally, Ag/Au alloy nanoparticles can be synthesized by co-reduction of a mixture precursor of Ag and Au, for example, HAuCl4 and AgNO3. Since the formation of Ag/Au alloy nanoparticles depends on the reduction rate of the respective precursors, it is difficult to achieve compositional homogeneity of Ag and Au across an alloy nanoparticle, as indicated by more close investigations, which reveal a stepwise reduction and growth of the metals. The homogeneity can be enhanced through interdiffusion of Ag and Au into one another by means of annealing at elevated temperatures, laser radiation or ultrasonic treatment. The energy input into the nanoparticles, however, is still limited in these cases, for example, the temperature of annealing is limited to the decomposition temperature of their capping agents (oleylamine, etc.) to ensure the stability of the nanoparticle colloid, which impedes Ag and Au from efficient interdiffusion and complete alloying.
Alloy nanostructures of Ag and Au may also arise from galvanic replacement of a Ag nanoparticle with a salt of Au, which, however, can often lead to formation of nanocages or nanoframes, as well as a lack of control over the ratio and the distribution of the Ag and Au components.
Compositional inhomogeneity is expected to be present in the Ag/Au alloy nanoparticles produced by the state-of-the-art synthesis strategies, with Ag/Au ratio varying from one domain to another. On one hand, as the stability of the alloy nanoparticles heavily relies on the Ag/Au ratio, corrosion easily starts from unstable domains upon exposure to an etchant, and the stability in both morphology and optical property of the alloy nanoparticles can be significantly reduced as a result. On the other hand, the compositional domains create interfaces, which may also affect the plasmonic property of the nanoparticles.
According to Mie theory, crystalline grain boundaries in a noble metal nanoparticle can play a critical role in damping of surface plasmons and significantly enhance scattering events. In a similar manner, damping of surface plasmons can be also resulted from interfaces between domains of different compositions and thus different dielectric properties in the Ag/Au alloy nanoparticles, producing broad bandwidths of the extinction spectrum as observed in most literature reports, which add additional limitations to many of their plasmon-based applications